Smith at Baylor has shown that when poly(A)+ RNA derived from human diploid fibroblasts at late passage is microinjected into the same cell type at early passage, the growth of the early passage cells is halted. A functionally identical activity derived from rat liver has a single molecular weight peak of activity on sucrose gradients. These data suggest that a single messenger RNA has the ability to shut off cell growth, and dilution experiments suggest that the message is abundant (1/100-1/1000 of total message). We plan to clone this mRNA by hybrid selection screening of a cDNA library, and then to analyze the structure and expression of the gene.